This invention relates generally to a steam generating system having a coal or oil fired boiler and a regenerative air preheater. More particularly, the present invention relates to a steam generating system having a boiler and a rotary regenerative air preheater.
During the combustion process in the boiler, the sulfur in the fuel is oxidized to SO2. After the combustion process, some amount of SO2 is further oxidized to SO3, with typical amounts on the order of 1 to 2% going to SO3. The presence of iron oxide, vanadium and other metals at the proper temperature range produces this oxidation. Selective catalytic reduction (SCR) are also widely known to oxidize a portion of the SO2 in the flue gas to SO3. The catalyst formulation (primarily the amount of vanadium in catalyst) impacts the amount of oxidation, with rates ranging from 0.5% to over 1.5%. Most typical is around 1%. Therefore plants firing a high sulfur coal with a new SCR can see a large increase in the SO3 emissions, which produce a visible plume, local acidic ground level problems and other environmental issues.
Regenerative air preheaters condense or trap a portion of the SO3 in the flue gas. This action depends primarily on the heat transfer plate temperatures, with the most critical parameter being the hottest temperature of the cold end elements. (The Air Preheaterxe2x80x94A Component for Emission Reduction (CO2+SO3), VGB Kraftwerkstechnik 75 (1995), Number 11). If one wants to capture a large amount of SO3 then the hottest or highest minimum cold end metal temperature (the temperature at the extreme gas out end of the element, assuming that each heat exchange element layer has the same element and gauge) must be below the acid dew point by some margin. In addition, coal having more alkaline fly ash (i.e., those with higher concentrations of MgO and CaO), allow operation at lower temperatures without experiencing heavy fouling.
For existing power plants it is often impossible, or very costly, to reduce the element temperatures to the level required to obtain sufficient removal of the SO3. The installed regenerative air preheater has a fixed depth for containing heat exchange elements that places a limitation on the amount of heat recovery possible (hence limitation to cold end metal temperature reduction). If additional surface area is installed within the existing air preheater, limitations of increased pressure drop and too small a hydraulic diameter are reached. A small hydraulic diameter is equivalent to a smaller flow area that is more prone to pluggage and results in faster pressure drop increases when fixed amounts of fouling deposits occur.
Briefly stated, the invention in a preferred form is an air preheater for use with a boiler which discharges a flow of flue gas containing SO3. The air preheater comprises a housing having oppositely disposed hot and cold ends. The hot end includes an air outlet duct and a flue gas inlet duct, the air outlet duct having a combustion air duct and an excess air duct. The cold end includes an air inlet duct and a flue gas outlet duct. The air inlet duct provides fluid communications with the atmosphere for receiving a flow of air. The combustion air duct provides fluid communications with the boiler for supplying a flow of combustion air to the boiler and the excess air duct provides fluid communications with the atmosphere for discharging a flow of excess air. The flue gas inlet duct provides fluid communications with the boiler for receiving the flow of flue gas and the flue gas outlet duct provides fluid communications with the atmosphere for discharging the flue gas. The air preheater also comprises a rotor rotatably mounted in the housing. The rotor includes an air sector in fluid communication with the air inlet and outlet ducts and a flue gas sector in fluid communication with the flue gas inlet and outlet ducts. At least one sector plate separates the air sector from the flue gas sectors. A plurality of heat exchange element basket assemblies rotate through the air and flue gas sectors, with the heat exchange element basket assemblies absorbing heat from the flow of flue gas in the flue gas sector and discharging heat to the flow of air in the air sector. The flow of excess air cools the heat exchange basket element assemblies below the dew point of the SO3, thereby condensing the SO3 for removal from the flow of flue gas.
The air outlet duct also has a wall separating the combustion air duct from the excess air duct. The wall also divides the air sector of the rotor into first and second segments, with the combustion air duct receiving the air flow through the first segment and the excess air duct receiving the air flow through the second segment. Each of the heat exchange element basket assemblies rotating out of the flue gas sector first enters the first segment of the air sector and then enters the second segment of the air sector.
A boiler system having the boiler and air preheater described above also includes a stack and an outlet ductwork providing fluid communication between the flue gas outlet duct of the air preheater and the stack. The outlet ductwork may discharge the flow of excess air directly to atmosphere. Alternatively, the excess air duct may be in fluid communication with the outlet ductwork to discharge the flow of excess air through the stack.
The boiler system also includes an inlet ductwork providing fluid communication between the boiler and the flue gas inlet duct of the air preheater. The boiler system may include apparatus for injecting an alkaline material into the inlet ductwork. Alternatively, the boiler system may include apparatus for injecting a precursor to an alkaline material into the boiler.
The boiler system includes a first particulate removal device positioned between the air preheater and the stack, dividing the outlet ductwork into a first segment extending between the air preheater and the first particulate removal device and a second segment extending between the first particulate removal device and the stack. The excess air duct may be in fluid communication with the first segment of the outlet ductwork. Alternatively, the boiler system may include a second particulate removal device in fluid communication with the excess air duct and the second segment of the outlet ductwork.
It is an object of the invention to provide a steam generating system which removes a large percentage of flue gas borne SO3 in the existing regenerative air preheater.
It is also an object of the invention to provide a steam generating system which minimizes fouling and corrosion problems associated with the removal of flue gas borne SO3.